RSK 1 (p90-RSK 1) is a member of a family of 90 kDa ribosomal protein S6 kinases, which includes RSK 1, RSK 2 and RSK 3. These are broadly expressed serine/threonine protein kinases (1) activated in response to mitogenic stimuli, including extracellular signal-regulated protein kinases Erk1 and Erk2 (2). p90-RSK 1 is activated by MAPK in vitro and in vivo via phosphorylation (3). Active RSKs appear to play a major role in transcriptional regulation by translocating to the nucleus and phosphorylating c-Fos and CREB (2,4).